Multi-functional active matrix organic light-emitting diode display

ABSTRACT

A multi-functional active matrix display comprises a transparent front sheet, a semi-transparent layer of light emissive devices adjacent the rear side of the front sheet and forming a matrix of display pixels, and a solar cell layer located behind the light emissive devices for converting both ambient light and internal light7 from the light emissive devices into electrical energy, the solar cell layer including an array of electrodes on the front surface of the solar cell layer for use in detecting the location of a change in the amount of light impinging on a portion of the front surface of the solar cell layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 13/689,241, filed Nov. 29, 2012, now allowed, whichclaims priority to U.S. Provisional Application No. 61/564,634 filedNov. 29, 2011, each of which is hereby incorporated by reference hereinin its entirety.

FIELD OF THE INVENTION

The present invention relates generally to active matrix displaysorganic light-emitting diode (AMOLED) displays and, more particularly,to multi-functional AMOLED displays having integrated solar cells.

BACKGROUND OF THE INVENTION

Organic light-emitting diodes (OLEDs) have attracted significantattention because of their great potential for making next-generationflat panel displays, i.e., active-matrix organic light-emitting diodedisplays (AMOLEDs). The standard OLEDs consist of a transparentelectrode, organic layers and a metallic reflective electrode, whichexperience a strong ambient light reflection and a reduced contrast ofdisplays operating under sunlight, mandating a circular polarizer. Otherapproaches, such as black cathodes, have been proposed. However, allthese designs absorb and waste the incident ambient light and asignificant amount of light emitted by the OLEDs. In addition, AMOLEDstoday are typically integrated with a touch panel in front of the AMOLEDin many applications such as cellular phones, personal digitalassistants (PDAs), computer displays and touch tablets. The light outputfrom the OLEDs is further reduced by the touch panel. Altogether, theAMOLED display system is complex and costly, and exhibits relatively lowefficiency and wastes a large amount of energy.

SUMMARY OF THE INVENTION

In accordance with one embodiment, a multi-functional active matrixdisplay comprises a transparent front sheet, a semi-transparent layer oflight emissive devices adjacent the rear side of the front sheet andforming a matrix of display pixels, and a solar cell layer locatedbehind the light emissive devices for converting both ambient light andinternal light from the light emissive devices into electrical energy,the solar cell layer including an array of electrodes on the frontsurface of the solar cell layer for use in detecting the location of achange in the amount of light impinging on a portion of the frontsurface of the solar cell layer.

In one implementation, the semi-transparent layer of light emissivedevices includes a substantially transparent anode adjacent thetransparent front sheet, a semi-transparent semiconductor stack formingorganic light emitting diodes adjacent the rear side of the anode, and asemi-transparent cathode adjacent the rear side of the semiconductorstack. A cover glass is spaced rearwardly of the cathode, covering thesolar cell layer, and a peripheral sealant bonds the cover glass to thefront sheet. The peripheral sealant holds the cover sheet spaced awayfrom the rear side of said cathode to form an air gap between thecathode and said the sheet.

This integration of the semi-transparent OLED panel and the solar paneloffers ease of fabrication and low cost while providing advantages ofmulti-functionality such as low ambient light reflection, low powerconsumption, high yield and optical touch screen functionality.Specifically, the integrated device permits the touching of portions ofthe front surface of an active matrix OLED display to be sensed by usingthe solar panel to produce an electrical output signal corresponding tolight that passes through said OLED display, including light reflectedfrom an object touching the front surface of said OLED display. Then thelocation of the touching object can be determined by monitoring andanalyzing the electrical output signals produced by said solar panel,using conventional touch-screen circuitry.

With this design, the incident ambient light and the portion of lightemitted through the transparent electrode is absorbed by the back solarcell instead of being reflected, resulting in improved contrast in thedisplayed image. Moreover, in contrast with the polarizer and blackcathode approaches, the absorbed light is recycled as useful electricalpower or touch feedback instead of being converted to heat. At the sametime, the display system can be manufactured with high yields by usingindependent panel and solar cell processes. Since the polarizer used inAMOLED displays typically has only about 43% transmittance, displayslose 57% of their electroluminance by using the polarizer.

Using the display system itself as an optical-based touch screen,without extra IR-LEDs and sensors, reduces the device complexity andfabrication cost of the display system by eliminating the need for extraIR-LEDs and sensors. The solar cell can also be used for initialuniformity calibration of the display, e.g., for correctingnon-uniformities of an active matrix OLED display by (a) integrating theOLED display with a solar panel that produces an electrical outputsignal corresponding to light emissions from the OLED display, (b)detecting non-uniformities in the OLED display from said output signal,and (c) correcting the detected non-uniformities in the OLED display.

This display system has a simple structure that exhibits superiorcontrast without sacrificing electroluminance, a high power efficiencyby recycling OLED internal emissions as well as incident ambient light,and touch screen functionality.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the present disclosure will becomeapparent upon reading the following detailed description and uponreference to the drawings.

FIG. 1 is diagrammatic side elevation of an active matrix display thatincludes integrated solar cell and semi-transparent OLED layers.

FIG. 2 is a plot of current efficiency vs. current density for theintegrated device of FIG. 1 and a reference device.

FIG. 3 is a plot of current efficiency vs. voltage for the integrateddevice of FIG. 1 with the solar cell in a dark environment, underillumination of the OLED layer, and under illumination of both the OLEDlayer and ambient light.

FIG. 4 is a diagrammatic illustration of the integrated device of FIG. 1operating as an optical-based touch screen.

FIG. 5 is a plot of current efficiency vs. voltage for the integrateddevice of FIG. 1 with the solar cell in a dark environment, underillumination of the OLED layer with and without touch.

FIG. 6A is an image of an AMOLED panel without compensation.

FIG. 6B is an image of an AMOLED panel with in-pixel compensation.

FIG. 6C is an image of an AMOLED panel with extra external calibration.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Although the invention will be described in connection with certainaspects and/or embodiments, it will be understood that the invention isnot limited to those particular aspects and/or embodiments. On thecontrary, the invention is intended to cover all alternatives,modifications, and equivalent arrangements as may be included within thespirit and scope of the invention as defined by the appended claims.

FIG. 1 illustrates a display system that includes a semi-transparentOLED layer 10 integrated with a solar panel 11 separated from the OLEDlayer 10 by an air gap 12. The OLED layer 10 includes multiple pixelsarranged in an X-Y matrix that is combined with programming, driving andcontrol lines connected to the different rows and columns of the pixels.A peripheral sealant 13 (e.g., epoxy) holds the two layers 10 and 11 inthe desired positions relative to each other. The OLED layer 10 has aglass substrate 14, the solar panel 11 has a glass cover 15, and thesealant 13 is bonded to the opposed surfaces of the substrate 14 and thecover 15 to form an integrated structure.

The OLED layer 10 includes a substantially transparent anode 20, e.g.,indium-tin-oxide (ITO), adjacent the glass substrate 14, an organicsemiconductor stack 21 engaging the rear surface of the anode 20, and acathode 22 engaging the rear surface of the stack 21. The cathode 22 ismade of a transparent or semi-transparent material, e.g., thin silver(Ag), to allow light to pass through the OLED layer 10 to the solarpanel 11. (The anode 20 and the semiconductor stack 21 in OLEDs aretypically at least semi-transparent, but the cathode in previous OLEDshas often been opaque and sometimes even light-absorbing to minimize thereflection of ambient light from the OLED.)

Light that passes rearwardly through the OLED layer 10, as illustratedby the right-hand arrow in FIG. 1, continues on through the air gap 12and the cover glass cover 15 of the solar cell 11 to the junctionbetween n-type and p-type semiconductor layers 30 and 31 in the solarcell. Optical energy passing through the glass cover 15 is converted toelectrical energy by the semiconductor layers 30 and 31, producing anoutput voltage across a pair of output terminals 32 and 33. The variousmaterials that can be used in the layers 30 and 31 to convert light toelectrical energy, as well as the material dimensions, are well known inthe solar cell industry. The positive output terminal 32 is connected tothe n-type semiconductor layer 30 (e.g., copper phthalocyanine) by frontelectrodes 34 attached to the front surface of the layer 30. Thenegative output terminal 33 is connected to the p-type semiconductorlayer 31 (e.g., 3, 4, 9, 10-perylenetetracarboxylic bis-benzimidazole)by rear electrodes 35 attached to the rear surface of the layer 31.

One or more switches may be connected to the terminals 32 and 33 topermit the solar panel 11 to be controllably connected to either (1) anelectrical energy storage device such as a rechargeable battery or oneor more capacitors, or (2) to a system that uses the solar panel 11 as atouch screen, to detect when and where the front of the display is“touched” by a user.

In the illustrative embodiment of FIG. 1, the solar panel 11 is used toform part of the encapsulation of the OLED layer 10 by forming the rearwall of the encapsulation for the entire display. Specifically, thecover glass 15 of the solar cell array forms the rear wall of theencapsulation for the OLED layer 10, the single glass substrate 14 formsthe front wall, and the perimeter sealant 13 forms the side walls.

One example of a suitable semitransparent OLED layer 10 includes thefollowing materials:

Anode 20

-   -   ITO (100 nm)

Semiconductor Stack 21

-   -   hole transport layer—N,N′-bis(naphthalen-1-yl)-N,N′-bis        (phenyl)benzidine (NBP) (70 nm)    -   emitter layer—tris(8-hydroxyquinoline) aluminum (Alq₃):        10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,        5H, 11H, [1] benzo-pyrano [6,7,8-ij]quinolizin-11-one (C545T)        (99%:1%) (30 nm)    -   electron transport layer—Alq3 (40 nm)    -   electron injection layer—4,7-diphenyl-1,10-phenanthroline        (Bphen): (Cs2CO3) (9:1) (10 nm)

Semitransparent Cathode 22—

-   -   MoO3:NPB(1:1) (20 nm)    -   Ag (14 nm)    -   MoO3:NPB(1:1) (20 nm)

The performance of the above OLED layer in an integrated device using acommercial solar panel was compared with a reference device, which wasan OLED with exactly the same semiconductor stack and a metallic cathode(Mg/Ag). The reflectance of the reference device was very high, due tothe reflection of the metallic electrode; in contrast, the reflectanceof the integrated device is very low. The reflectance of the integrateddevice with the transparent electrode was much lower than thereflectances of both the reference device (with the metallic electrode)and the reference device equipped with a circular polarizer.

The current efficiency-current density characteristics of the integrateddevice with the transparent electrode and the reference device are shownin FIG. 2. At a current density of 200 A/m², the integrated device withthe transparent electrode had a current efficiency of 5.88 cd/A, whichwas 82.8% of the current efficiency (7.1 cd/A) of the reference device.The current efficiency of the reference device with a circular polarizerwas only 60% of the current efficiency of the reference device. Theintegrated device converts both the incident ambient light and a portionof the OLED internal luminance into useful electrical energy instead ofbeing wasted.

For both the integrated device and the reference device described above,all materials were deposited sequentially at a rate of 1-3 Å/s usingvacuum thermal evaporation at a pressure below 5×10⁻⁶ Torr on ITO-coatedglass substrates. The substrates were cleaned with acetone and isopropylalcohol, dried in an oven, and finally cleaned by UV ozone treatmentbefore use. In the integrated device, the solar panel was a commercialSanyo Energy AM-1456CA amorphous silicon solar cell with a short circuitcurrent of 6 μA and a voltage output of 2.4V. The integrated device wasfabricated using the custom cut solar cell as encapsulation glass forthe OLED layer.

The optical reflectance of the device was measured by using a ShimadzuUV-2501PC UV-Visible spectrophotometer. The current density(J)-luminance (L)-voltage (V) characteristics of the device was measuredwith an Agilent 4155C semiconductor parameter analyzer and a siliconphotodiode pre-calibrated by a Minolta Chromameter. The ambient lightwas room light, and the tests were carried out at room temperature. Theperformances of the fabricated devices were compared with each other andwith the reference device equipped with a circular polarizer.

FIG. 3 shows current-voltage (I-V) characteristics of the solar panel(1) in dark, (20 under the illumination of OLED, and (3) underillumination of both ambient light and the OLED at 20 mA/cm². The darkcurrent of the solar cell shows a nice diode characteristic. When thesolar cell is under the illumination of the OLED under 20 mA/cm² currentdensity, the solar cell shows a short circuit current (I_(sc)) of −0.16μA, an open circuit voltage (V_(oc)) of 1.6V, and a filling factor (FF)of 0.31. The maximum converted electrical power is 0.08 μW, whichdemonstrates that the integrated device is capable of recycling aportion of the internal OLED luminance energy. When the solar cell isunder the illumination of both ambient light and the overlying OLED, thesolar cell shows a short circuit current (I_(sc)) of −7.63 μA, an opencircuit voltage (V_(oc)) of 2.79V, and a filling factor (FF) of 0.65.The maximum converted electrical power is 13.8 μW in this case. Theincreased electrical power comes from the incident ambient light.

Overall, the integrated device shows a higher current efficiency thanthe reference device with a circular polarizer, and further recycles theenergy of the incident ambient light and the internal luminance of thetop OLED, which demonstrates a significant low power consumption displaysystem.

Conventional touch displays stack a touch panel on top of an LCD orAMOLED display. The touch panel reduces the luminance output of thedisplay beneath the touch panel and adds extra cost to the fabrication.The integrated device described above is capable of functioning as anoptical-based touch screen without any extra panels or cost. Unlikeprevious optical-based touch screens which require extra IR-LEDs andsensors, the integrated device described here utilizes the internalillumination from the top OLED as an optical signal, and the solar cellis utilized as an optical sensor. Since the OLED has very good luminanceuniformity, the emitted light is evenly spread across the device surfaceas well as the surface of the solar panel. When the front surface of thedisplay is touched by a finger or other object, a portion of the emittedlight is reflected off the object back into the device and onto thesolar panel, which changes the electrical output of the solar panel. Thesystem is able to detect this change in the electrical output, therebydetecting the touch. The benefit of this optical-based touch system isthat it works for any object (dry finger, wet finger, gloved finger,stylus, pen, etc.), because detection of the touch is based on theoptical reflection rather than a change in the refractive index,capacitance or resistance of the touch panel.

FIG. 4 is a diagrammatic illustration of the integrated device of FIG. 1being used as a touch screen. To allow the solar cell to convert asignificant amount of light that impinges on the front of the cell, thefront electrodes 34 are spaced apart to leave a large amount of openarea through which impinging light can pass to the front semiconductorlayer 30. The illustrative electrode pattern in FIG. 4 has all the frontelectrodes 34 extending in the X direction, and all the back contacts 35extending in the Y direction. Alternatively, one electrode can bepatterned in both directions. An additional option is the addition oftall wall traces covered with metal so that they can be connected to theOLED transparent electrode to further reduce the resistance. Anotheroption is to fill the gap 12 between the OLED layer 10 and the coverglass 15 with a transparent material that acts as an optical glue, forbetter light transmittance.

When the front of the display is touched or obstructed by a finger 40(FIG. 4) or other object that reflects or otherwise changes the amountof light impinging on the solar panel at a particular location, theresulting change in the electrical output of the solar panel can bedetected. The electrodes 34 and 35 are all individually connected to atouch screen controller circuit that monitors the current levels in theindividual electrodes, and/or the voltage levels across different pairsof electrodes, and analyzes the location responsible for each change inthose current and/or voltage levels. Touch screen controller circuitsare well known in the touch-screen industry, and are capable of quicklyand accurately reading the exact position of a “touch” that causes achange in the electrode currents and/or voltages being monitored. Thetouch screen circuits may be active whenever the display is active, or aproximity switch can be sued to activate the touch screen circuits onlywhen the front surface of the display is touched.

The solar panel may also be used for imaging, as well as a touch screen.An algorithm may be used to capture multiple images, using differentpixels of the display to provide different levels of brightness forcompressive sensing.

FIG. 5 is a plot of normalized current I_(sc) vs. voltage V_(oc)characteristics of the solar panel under the illumination of theoverlying OLED layer, with and without touch. When the front of theintegrated device is touched, I_(sc) and V_(oc) of the solar cell changefrom −0.16 μA to −0.87 μA and 1.6 V to 2.46 V, respectively, whichallows the system to detect the touch. Since this technology is based onthe contrast between the illuminating background and the light reflectedby a fingertip, for example, the ambient light has an influence on thetouch sensitivity of the system. The changes in I_(sc) or V_(oc) in FIG.5 are relatively small, but by improving the solar cell efficiency andcontrolling the amount of background luminance by changing the thicknessof the semitransparent cathode of the OLED, the contrast can be furtherimproved. In general, a thinner semitransparent OLED cathode willbenefit the luminance efficiency and lower the ambient lightreflectance; however, it has a negative influence on the contrast of thetouch screen.

In a modified embodiment, the solar panel is calibrated with differentOLED and/or ambient brightness levels, and the values are stored in alookup table (LUT). Touchiong the surface of the display changes theoptical behavior of the stacked structure, and an expected value foreach cell can be fetched from the LUT based on the OLED luminance andthe ambient light. The output voltage or current from the solar cellscan then be read, and a profile created based on differences betweenexpected values and measured values. A predefined library or dictionarycan be used to translate the created profile to different gestures ortouch functions.

In another modified embodiment, each solar cell unit represents a pixelor sub-pixel, and the solar cells are calibrated as smaller units (pixelresolution) with light sources at different colors. Each solar cell unitmay represent a cluster of pixels or sub-pixels. The solar cells arecalibrated as smaller units (pixel resolution) with reference lightsources at different color and brightness levels, and the values storedin LUTs or used to make functions. The calibration measurements can berepeated during the display lifetime by the user or at defined intervalsbased on the usage of the display. Calibrating the input video signalswith the values stored in the LUTs can compensate for non-uniformity andaging. Different gray scales may be applied while measuring the valuesof each solar cell unit, and storing the values in a LUT.

Each solar cell unit can represent a pixel or sub-pixel. The solar cellcan be calibrated as smaller units (pixel resolution) with referencelight sources at different colors and brightness levels and the valuesstored in LUTs or used to make functions. Different gray scales may beapplied while measuring the values of each solar cell unit, and thencalibrating the input video signals with the values stored in the LUTsto compensate for non-uniformity and aging. The calibration measurementscan be repeated during the display lifetime by the user or at definedintervals based on the usage of the display.

Alternatively, each solar cell unit can represent a pixel or sub-pixel,calibrated as smaller units (pixel resolution) with reference lightsources at different colors and brightness levels with the values beingstored in LUTs or used to make functions, and then applying differentpatterns (e.g., created as described in U.S. Patent ApplicationPublication No. 2011/0227964, which is incorporated by reference in itsentirety herein) to each cluster and measuring the values of each solarcell unit. The functions and methods described in U.S. PatentApplication Publication No. 2011/0227964 may be used to extract thenon-uniformities/aging for each pixel in the clusters, with theresulting values being stored in a LUT. The input video signals may thenbe calibrated with the values stored in LUTs to compensate fornon-uniformity and aging. The measurements can be repeated during thedisplay lifetime either by the user or at defined intervals based ondisplay usage.

The solar panel can also be used for initial uniformity calibration ofthe display. One of the major problems with OLED panels isnon-uniformity. Common sources of non-uniformity are the manufacturingprocess and differential aging during use. While in-pixel compensationcan improve the uniformity of a display, the limited compensation levelattainable with this technique is not sufficient for some displays,thereby reducing the yield. With the integrated OLED/solar panel, theoutput current of the solar panel can be used to detect and correctnon-uniformities in the display. Specifically, calibrated imaging can beused to determine the luminance of each pixel at various levels. Thetheory has also been tested on an AMOLED display, and FIG. 6 showsuniformity images of an AMOLED panel (a) without compensation, (b) within-pixel compensation and (c) with extra external compensation. FIG.6(c) highlights the effect of external compensation which increases theyield to a significantly higher level (some ripples are due to theinterference between camera and display spatial resolution). Here thesolar panel was calibrated with an external source first and then thepanel was calibrated with the results extracted from the panel.

As can be seen from the foregoing description, the integrated displaycan be used to provide AMOLED displays with a low ambient lightreflectance without employing any extra layers (polarizer), low powerconsumption with recycled electrical energy, and functionality as anoptical based touch screen without an extra touch panel, LED sources orsensors. Moreover, the output of the solar panel can be used to detectand correct the non-uniformity of the OLED panel. By carefully choosingthe solar cell and adjusting the semitransparent cathode of the OLED,the performance of this display system can be greatly improved.

While particular implementations and applications of the presentdisclosure have been illustrated and described, it is to be understoodthat the present disclosure is not limited to the precise constructionand compositions disclosed herein and that various modifications,changes, and variations can be apparent from the foregoing descriptionswithout departing from the spirit and scope of the invention as definedin the appended claims.

What is claimed is:
 1. A method of sensing a touching of portions of thefront surface of an active matrix OLED display, the method comprising:integrating the OLED display with a solar panel located behind said OLEDdisplay and producing an electrical output signal corresponding to lightthat passes through said OLED display, including light emitted by saidOLED display and reflected from an object touching the front surface ofsaid OLED display; and detecting the location of said touching objectfrom said electrical output signal produced by said solar panel.
 2. Anactive matrix display comprising: a transparent front sheet, asemi-transparent layer of light emissive devices adjacent the rear sideof said front sheet and forming a matrix of display pixels fordisplaying images for viewing through said front sheet, and a solarpanel located behind said light emissive devices, said solar panelincluding an array of electrodes on at least one surface of said solarpanel for generating output signals for use in detecting the location ofa change in the amount of light impinging on a portion of the frontsurface of said solar panel due to an object in front of said frontsheet reflecting light from said emissive devices back through saidfront sheet and said emissive devices to said solar panel.
 3. The activematrix display of claim 2 in which said semi-transparent layer of lightemissive devices includes a substantially transparent anode adjacentsaid front sheet, a semi-transparent semiconductor stack forming organiclight emitting diodes adjacent the rear side of said anode, asemi-transparent cathode adjacent the rear side of said semiconductorstack, a cover glass spaced rearwardly of said cathode and covering saidsolar panel, and a peripheral sealant bonding said cover glass to saidtransparent front sheet.
 4. The active matrix display of claim 3 inwhich said peripheral sealant holds said cover glass spaced away fromthe rear side of said cathode to form an air gap between said cathodeand said cover glass.
 5. The active matrix display of claim 2 in whichsaid transparent front sheet is a glass substrate for saidsemi-transparent layer of light emissive devices.
 6. The active matrixdisplay of claim 2 in which said solar panel includes laminated N-typeand P-type semiconductor materials, and said array of electrodesincludes multiple electrode segments located on the front surface ofsaid semiconductor laminate and spaced from each other to allow lightfrom said light emissive devices to impinge on said semiconductorlaminate.
 7. The active matrix display of claim 6 in which said array ofelectrodes includes first spaced electrode segments running in a firstdirection on one surface of said semiconductor laminate, and secondspaced electrode segments running in a second direction on the oppositesurface of said semiconductor laminate.
 8. The active matrix display ofclaim 2 in which said semi-transparent layer of light emissive devicesincludes a substantially transparent anode and a semi-transparentcathode.
 9. The active matrix display of claim 2 further comprising: atouch screen circuit coupled to said solar panel for receiving saidoutput signals from said array of electrodes and for detecting thelocation of said change in the amount of light with use of said outputsignals.
 10. An active matrix display system comprising: a transparentfront sheet, a semi-transparent layer of light emissive devices adjacentthe rear side of said front sheet and forming a matrix of display pixelsfor displaying images for viewing through said front sheet, a solarpanel located behind said light emissive devices for converting lightemitted from said light emissive devices into electrical energy, andelectrical energy storage coupled to said solar panel for storing saidelectrical energy.
 11. The active matrix display of claim 10 in whichsaid semi-transparent layer of light emissive devices includes asubstantially transparent anode adjacent said front sheet, asemi-transparent semiconductor stack forming organic light emittingdiodes adjacent the rear side of said anode, a semi-transparent cathodeadjacent the rear side of said semiconductor stack, a cover glass spacedrearwardly of said cathode and covering said solar panel, and aperipheral sealant bonding said cover glass to said transparent frontsheet.
 12. The active matrix display of claim 11 in which saidperipheral sealant holds said cover glass spaced away from the rear sideof said cathode to form an air gap between said cathode and said coverglass.
 13. The active matrix display of claim 10 in which saidtransparent front sheet is a glass substrate for said semi-transparentlayer of light emissive devices.
 14. The active matrix display of claim10 in which said solar panel includes laminated N-type and P-typesemiconductor materials.
 15. The active matrix display of claim 10 inwhich said semi-transparent layer of light emissive devices includes asubstantially transparent anode and a semi-transparent cathode.